Relationship between brake system vibrations and particulate matter emissions in a reduced-scale dynamometer

Brake particulate emissions and brake system vibrations are major concerns in the brake industry. Vibrational phenomena such as brake squeal and creep groan, together with particulate emissions, are currently among the most relevant topics in brake research. Recent studies suggest that vibration analysis and particulate emission measurements should be integrated within the same experimental campaigns. Simplified test systems, such as pin-on-disc tribometers, have been extensively used to investigate these phenomena due to their cost-effectiveness and the high control over test conditions. However, system vibrations and particulate emissions have often been analysed separately. Compared to a pin-on-disc tribometer, the reduced-scale inertial dynamometer represents a more advanced testing system, as it allows testing under controlled pressure, temperature, and sliding speed while maintaining equivalence with real vehicle braking conditions. In the literature, reduced-scale dynamometers have primarily been used to characterize braking performance and particulate emissions of the friction materials. However, a detailed multi-factor comparison between pin-on-disc and reduced-scale dynamometer tests is missing, despite its importance for understanding their results. In addition, a systematic study of the squeal propensity of the reduced-scale inertial dynamometer is still lacking, even though it has been documented in the literature for pin-on-disc systems. Furthermore, research on vibration–emission relationships in dynamometers remains limited, as these correlations have mostly been investigated using pin-on-disc setups for limited testing conditions. Considering these research gaps, this PhD work examines the complex relationships among frictional behaviour, system dynamics, vibrations, and particulate emissions using a reduced-scale inertial dynamometer and a commercial low-metallic friction material. As a first step, a comparative analysis was conducted between a pin-on-disc tribometer and a reduced-scale dynamometer to assess the consistency of key parameters, including friction coefficient, wear rate, particulate emissions, and system vibrations. The results confirmed that the two systems showed comparable friction coefficients and wear regimes. However, the pin-on-disc exhibited higher vibration intensities, mainly due to its lower structural stiffness. Particulate concentrations measured on the dynamometer presented a stronger correlation with peak emissions observed during pin-on-disc testing, highlighting the importance of considering the transient phases when evaluating friction materials with simplified tribometers. Secondly, the influence of disc boundary conditions on the squeal propensity of the reduced-scale inertial dynamometer was investigated. The testing rig was adapted to run the industrial SAE J2521 test procedure. Experimental modal analysis revealed that the less constrained disc–shaft configuration exhibited reduced structural stiffness, an average decrease in modal damping of about 25%, and the occurrence of two additional squeal frequencies. These were supplemented by sound pressure level increases of up to 20 dB(A) at the first squeal frequency, making the behaviour of the reduced-scale dynamometer more representative of a full-scale braking system. The study confirmed that mode coupling is the dominant mechanism responsible for squeal generation and that damping homogeneity among brake components is fundamental for controlling squeal propensity. Compared to other simplified systems reported in the literature, the present setup showed a greater separation between the disc and pad resonance frequencies, likely due to a lower mass ratio between the two components. Finally, the reduced-scale dynamometer was instrumented with an optical particle sizer to evaluate the effect of the same system modification on particulate emissions. Despite similar friction coefficients and comparable friction layer extension on the pads’ surfaces, the more flexible configuration emitted approximately 10% more particles. Frequency-domain analysis revealed localized increases in vibrational energy below 1200 Hz and decreases above 10 kHz in the less stiff configuration. These findings suggest that lower structural stiffness can amplify local contact instabilities and facilitate the disruption of the friction layer, even in the absence of significant variations in friction coefficient. Overall, this work suggests that the structural dynamics of braking systems play a key role not only in brake vibrations but also in non-exhaust particulate emissions. The outcomes confirm the reduced-scale dynamometer as a useful tool for early-stage research of brake systems.